Treasurys have a new rival for safe haven status: US companies.

Bonds of Exxon Mobil and Johnson & Johnson are trading with yields below those of comparable Treasurys, a sign that investors perceive them as a safer bet.

It is a rare phenomenon that some market observers said could be the beginning of a new era for debt markets. It could ultimately mean some companies will borrow at lower rates than the US government.

For now, just a handful of relatively short-term bonds yield less than comparable Treasury bonds. But some market observers said some fundamental changes in the financial health of US companies relative to the government, including the fact that some corporations are more highly rated than Uncle Sam, suggest it could become a longer lasting trend.

"If it grows to be more like dozens of issues, then it stops becoming an anomaly and it becomes a big deal," said James Bianco, founder of Bianco Research in Chicago. Bianco recalled a few similar instances in the past 30 years but none lasted long.

Since the turn of the 20th century, Treasurys have formed the benchmark for fixed rate US company debt. Companies and investors measure corporate borrowing rates relative to what the government pays. But several forces are combining to upend that market convention.

Money is pouring into highly rated US corporate bonds at an even faster pace than Treasury debt, according to fund tracker Lipper, as buyers clamour for investments perceived as safe that typically also yield a bit more than Treasurys.

A shrinking supply of top-rated debt is also driving investors toward the highest rated US companies. That demand has driven up bond prices and pushed down yields.

The soaring demand has enabled companies of all sizes to raise $1.2 trillion from issuing debt this year, already the busiest year on record, according to data provider Dealogic. US corporations, now sitting on $1.73 trillion of cash and borrowing at the lowest rates in history, are arguably in the best shape ever.

Should the economy continue to pick up steam, US companies will become even healthier, some argue. By contrast, the Treasury is burdened with more debt than ever, and government deficits are likely to keep increasing for the foreseeable future.

The prospect of further upheaval, as Congress battles whether to extend a raft of tax cuts and clip spending, is adding to the uncertainty. Some investors worry that the country could face a second credit rating downgrade, following Standard & Poor's cut below triple-A last year.

"Every day the debt and credit profile of the US gets worse, yet you have corporate America that is lean and mean," said Jason Graybill, senior managing director at Carret, who oversees a $1.2bn bond portfolio and sold Treasurys 18 months ago to pile into more corporate debt. "It's only a matter of time" before companies offer new debt below Treasury yields, he said.

Still, some observers argue that yields on corporate bonds could soon rise. They said the bonds of Exxon and Johnson & Johnson are short-term bonds that are inherently volatile. And others said US government debt is still widely seen as the safest debt available. The market is larger and much more active, making it more attractive than corporate debt to some investors.

"The Treasury has an unlimited printing press and there will always be demand," said Kam Poon, portfolio manager of short-term fixed income strategies at Aberdeen Asset Management in Philadelphia.

Poon, who oversees $1.1bn of bonds, has sold corporate bonds in which the yields have been close to Treasurys, as well as securities whose yields have dipped below Treasurys.

The phenomenon has been seen in what is known as the secondary bond market, in which bonds are traded after they are first issued.

Bonds of Exxon coming due in 13 months were quoted on Tuesday at 0.01% less than the comparable Treasury, according to Benchmark Solutions, a pricing service.

Bonds of Johnson & Johnson due in May 2014 also recently traded at 0.01% less than Treasurys. Both are rated triple-A by S&P. Representatives for Exxon and Johnson & Johnson declined to comment.

In the new issue market, companies continue to set records. Microsoft on Friday sold five-year bonds at a yield of just 0.27% above comparable Treasurys, the narrowest premium on such debt going back to 1994, according to Dealogic.

People familiar with the sale said Microsoft wanted to take advantage of low rates. Microsoft will put the $2.25bn it raised toward early repayment of similar debt the company issued in February 2011; its new five-year debt Friday priced to yield 0.875% compared with 2.5% last year.

"These are truly unprecedented conditions for blue chip companies in the debt markets," said Brent Callinicos, treasurer of Internet search firm Google. Google bonds due in 2014 yield just 0.02% over Treasurys.

The phenomenon could become more widespread as investors continue to search among a shrinking pool of highly rated bonds. According to Credit Suisse, the share of worldwide government debt rated triple-A has fallen to 39% of the total from 58% in early 2010.

Corporate bonds "are definitely the new safe havens in this world," said Fer Koch, director of US credit strategy at Credit Suisse.

--Write to Patrick McGee and Katy Burne at patrick.mcgee@dowjones.com and katy.burne@dowjones.com